1. Field of the Invention
The present invention relates to heavy duty pneumatic radial ply tires suitable for use in heavy vehicles such as a truck, a bus and the like, and more particularly to an improvement of a pneumatic tire provided with a tread pattern divided by two main longitudinal grooves having deepest depth extending to the circumferential direction of said tread into a tread center portion and both shoulder portions in which a rib or block pattern is provided thereon.
2. Prior Art and Problems Thereof
In general, it has been well known that radial tires have several merits such as wear resistance of tread rubber, lack of heat generation during high speed traveling, puncture resistance and the like as compared with conventional bias tires. In consequence of the remarkable improvement of highways networks, the demand for heavy duty radial tires has been rapidly increased. On the other hand, the performance of vehicles has been improved, so that traveling distance of vehicles or cars have been considerably spread in conjunction with the development of the highways. Furthermore, when the length of highways grow longer, not only the chance of continuous traveling for a long period of time at high speed has been increased, but also the requirement for a tire having long wear life has been rapidly conspicuous for economic reasons.
However, there has been fatal problems in that the means according to the prior art to solve these problems are all antipathetic to each other in that a tire designed to weigh heavily in durability to high speed running is inevitably followed by short life because of wear and, on the contrary, a tire weighted in resistance to wear and long life is insufficient in durability to high speed running.
For instance, the means of the prior art for achieving improvement in durability to high speed running have been such means as the employment of a tread small in gauge for the need to decrease heat generation. To explain it cincretely, a tire in the size of 10,00 R 20 having a maximum speed capability of 80 km/h is 20.6 mm in maximum groove depth, on the contrary, a tire having a maximum speed capability of 100 km/h is 13.5 mm in maximum groove depth which is the category of "highway depth", thus, to compared with them, the difference in tread gauge is 7 mm at the difference of 20 km/h in maximum speed, the wear life of tread is reduced in keeping with the difference in tread gauge, accordingly the wear life of such tires having thin tread gauge is shortened as an inevitable consequence.
However, the user of tires cannot be satisfied with such unbalanced tire performance for any length of time, the requirement for well balanced tire, which has a long life and an improved high speed durability, has become conspicuous of late. That is, it is required to increase the durability of such tire as the depth of longitudinal main grooves is deeper than that of the conventional tire and the thickness of tread gauge is greater than that of a conventional one. Furthermore, as the problem inherent to a tire having a carcass ply of radial construction, there is a problem of wear which causes generation of steps extending transversely in the cross sectional direction of the tire and particularly causes the tread rubber facing both the shoulder portions to wear more readily than the tread rubber facing the center portion which is called uneven shoulder wear.
Many attempts such as means to increase the tread radius or to decrease the tread width have been made to avoid uneven shoulder of the tire, but hitherto none has led to fully satisfactory results which can effectively avoid uneven shoulder wear of the tire, by which the wear life of the tire is caused to be greatly shortened.